Ultrathin All-Angle Hyperbolic Metasurface Retroreflectors Based on Directed Routing of Canalized Plasmonics.

Retroreflectors that can accurately redirect the incident waves in free space back along their original channels provide unprecedented opportunities for light manipulation. However, to the best of our knowledge, they suffer from either the bulky size, narrow angular bandwidths, or time-consuming postprocessing, which essentially limits their further applications. Here, a scheme for designing ultrathin all-angle real-time retroreflectors based on hyperbolic plasmonic metasurfaces is proposed and experimentally demonstrated. The physical mechanism underlying the scheme is the orthogonality between the traveling waves in free space and the canalized spoof surface plasmon on the hyperbolic plasmonic metasurfaces, which guarantee their high-efficiency and all-angle mutual conversion. In this case, the strong confinement characteristic that benefited from the enhanced light-matter interaction enables us to route and retroreflect the canalized spoof surface plasmon with extremely thin structures. As proof of the scheme, a retroreflector prototype with a thickness approximately equal to the central wavelength is designed and fabricated. Further experimental investigation obtains a half-power field of view up to 53° and a maximum efficiency of 83.2%. This scheme can find promising applications in target detection, remote sensing, and diverse on-chip light control devices.

Terahertz non-label subwavelength imaging with composite photonics-plasmonics structured illumination.

Inspired by the capability of structured illumination microscopy (SIM) in subwavelength imaging, many researchers devoted themselves to investigating this methodology. However, due to the free-propagating feature of the traditional structured illumination fields, the resolution can be only improved up to two-fold of the diffraction-limited microscopy. Besides, most of the previous studies, relying on incoherent illumination sources, are restricted to fluorescent samples. In this work, a subwavelength non-fluorescent imaging method is proposed based on the illumination of terahertz traveling waves and plasmonics. Excited along with a metal grating, the spoof surface plasmons (SSPs) are employed as one of the illuminating sources. When the scattering waves with the SSPs illumination are captured, the sample's high-order spatial frequencies (SF) components are already encoded into the obtainable low-order ones. Then, a modified post-processing algorithm is exploited to shift the modulated SF components to their actual positions in the SF domain. In this manner, the fine information of samples is introduced to reconstruct the desired imaging, leading to an enhancement of the resolution up to 0.12λ0. Encouragingly, the resolution can be further enhanced by attaching extra illumination of SSPs with an elaborately selected frequency. This method holds promise for some important applications in terahertz non-fluorescent microscopy and sample detection with weak scattering.

Efficient magnetic-coupling excitation of LSSPs on high-Q multilayer planar-circular-grating resonators.

Recently, ultrathin localized spoof surface plasmon (LSSP) resonators are found to have intrinsic defects of relatively low quality factors (Q-factors) because of unavoidable material and radiation losses. In this paper, multilayer structures of planar-circular-grating resonators and their magnetic-coupling schemes are proposed to achieve effective excitation of high-Q LSSPs modes. By adopting the multilayer structures with air between the layers, the power dissipation effected by both material and radiation losses is significantly suppressed. Experimental results show that the Q-factors could reach more than 200 and the excitation efficiencies could reach more than 90%. Numerical simulations show the distribution of the electromagnetic field and illustrate the principle of magnetic coupling. Besides, the Q-factors of resonators with different structural parameters were measured and analyzed. This study aims to provide some inspirations on planar gyro-devices and to improve the performance of existing applications, such as sensors and filters.

Design of Double-Layer Electrically Extremely Small-Size Displacement Sensor.

In this paper, a displacement sensor with an electrically extremely small size and high sensitivity is proposed based on an elaborately designed metamaterial element, i.e., coupled split-ring resonators (SRRs). The sensor consists of a feeding structure with a rectangular opening loop and a sensing structure with double-layer coupled SRRs. The movable double-layer structures can be used to measure the relative displacement. The size of microwave displacement sensors can be significantly reduced due to the compact feeding and sensing structures. By adjusting the position of the split gap within the resonator, the detection directions of the displacement sensing can be further expanded accordingly (along with the x- or y-axis) without increasing its physical size. Compared with previous works, the extremely compact size of 0.05λ0 × 0.05λ0 (λ0 denotes the free-space wavelength), a high sensitivity, and a high quality factor (Q-factor) can be achieved by the proposed sensor. From the perspective of the advantages above, the proposed sensor holds promise for being applied in many high-precision industrial measurement scenarios.

Bifunctional Luneburg-fish-eye lens based on the manipulation of spoof surface plasmons.

Manipulation of spoof surface plasmons (SSPs) has recently intrigued enormous interest due to the capability of guiding waves with subwavelength footsteps. However, most of the previous studies, manifested for a single functionality, are not suitable for multifunctional integrated devices. Herein, a bifunctional Luneburg-fish-eye lens is proposed based on a 2D metal pillar array. First, by tuning the dimension of the metal pillars in the array, its ability to precisely manipulate the SSPs along one direction is confirmed, achieving subwavelength focusing and imaging with a resolution up to 0.14λ. Then, separately controlling the propagation of the SSPs along the orthotropic directions is further implemented, and the bifunctional Luneburg-fish-eye lens is realized. It is experimentally characterized as a Luneburg lens along the x axis, whereas in the y axis, it presents the properties of a Maxwell fish-eye lens. This bifunctional lens can reduce the system complexity and exert flexibility in multifunctional applications, while the proposed metal pillar-based design method broadens the application range of the gradient refractive-index lens in microwave, terahertz, and even optical ranges.

High-efficiency directional excitation of spoof surface plasmons by periodic scattering cylinders.

In this Letter, we propose and experimentally demonstrate a simple but efficient method to excite spoof surface plasmons (SSP) through periodic metallic cylinders at microwave frequencies. The rigorous multiple scattering theory indicates that most of the incident propagating waves can pass the cylinders and be converted into the desired harmonics. Furthermore, by tuning the incident angle, controlling the directions of the excited SSP at different frequencies is also realized. The numerical simulations achieve a bidirectional efficiency of 90% at 9.68 GHz and unidirectional efficiency of 79%-85% at 7.46-9.7 GHz, when the incident angle changes from 60° to 120°. Meanwhile, the maximum contrast ratio between the powers of SSP launched in two opposite directions can reach up to 34 dB. The experimental results under 90° and 77.5° illuminations at 9.68 and 8.56 GHz provide strong support for the coupling mechanism. This method may provide technique support in the SSP-based communication and imaging systems.

High-efficiency terahertz spin-decoupled meta-coupler for spoof surface plasmon excitation and beam steering.

Spoof surface plasmon (SSP) meta-couplers that efficiently integrate other diversified functionalities into a single ultrathin device are highly desirable in the modern microwave and terahertz fields. However, the diversified functionalities, to the best of our knowledge, have not been applied to circular polarization meta-couplers because of the spin coupling between the orthogonal incident waves. In this paper, we propose and demonstrate a terahertz spin-decoupled bifunctional meta-coupler for SSP excitation and beam steering. The designed meta-coupler is composed of a coupling metasurface and a propagating metasurface. The former aims at realizing anomalous reflection or converting the incident waves into SSP under the illumination of the right or left circular polarization waves, respectively, and the latter are used to guide out the excited SSP. The respective converting efficiency can reach 82% and 70% at 0.3THz for the right and left circular polarization incident waves. Besides, by appropriately adjusting the reflection phase distribution, many other functionalities can also be integrated into the meta-coupler. Our study may open up new routes for polarization-related SSP couplers, detectors, and other practical terahertz devices.

Terahertz multichannel metasurfaces with sparse unit cells.

Reflective multichannel metasurfaces are flat reflectors that can control incident and reflected waves in a number of propagating directions simultaneously. However, they are always densely discretized with a high spatial resolution, which increases the manufacturing complexity. In this Letter, to the best of our knowledge, a new method that combines the array antenna theory with the metagratings theory is proposed. We demonstrate that the unit cells with a linear gradient phase in each period of the metasurfaces can eliminate specific space harmonics. With this method, multichannel metasurfaces can be designed with sparse unit cells, and high efficiency is maintained simultaneously. As proofs of the method, we design three different terahertz multichannel metasurfaces with no more than three unit cells per period. The simplification of structures can efficiently reduce the manufacturing complexity. This work may open up new routes in designing multichannel metasurfaces.

Superfocusing of terahertz wave through spoof surface plasmons.

In this paper, we propose and numerically demonstrate a new way to realize superfocusing of terahertz waves via the spoof surface plasmons (SSP). With the assist of a modified subwavelength metallic grating, a near-field rapid oscillation can be formed, originating from the Fabry-Perot resonances due to the reflection of SSP waves at terminations. We show that the field pattern of oscillation on textured metallic surface can be engineered by adjusting groove width and grating number. This produces a desired modulation of phase and amplitude for the radiationless electromagnetic interference (REI) focusing. The effective focusing depth through the corrugated metal is evaluated by the full-width-half-maximum (FWHM) beamwidth. At the situation of third-order Fabry-Perot resonance, the FWMH reaches up to 0.069λ at a distance of 0.1λ, improving the beamwidth by more than 540% compared with a single slit. The FWHM is optimized to 0.06λ as the order of Fabry-Perot resonance becomes seven, leading to the superfocusing metric of 1.67. On the basis of this, we further show the focusing ability can be held on the ultra-thin metallic grating. Two-dimensional subwavelength focusing behavior is also numerically verified. Our study may extend the working distance of sensing and super-resolution imaging devices at terahertz frequency.